Some Faction Missions require you to fend off large groups of enemies, giving you a good opportunity to finish this achievement.
Faction mission from the Ular Boys: Holy Smoke
Coordinates: X:13860 Y:30596 (One of the southern islands)
This mission has you defending an opium stash at a temple site. Waves of enemies will come from 3 sides, with about 10-12 enemies per wave. Plenty of meat to grab this achievement
Note: If the mission isn't available, keep causing chaos until you unlock new faction missions. It will be there eventually.